A large increase in population, increased industrialization and rising amounts of traffic lead to a concentration of contaminants in the environmental air that has assumed critical proportions. In this connection, nitrogen oxide emissions are of special importance, and these may be attributed in large measure to the combustion of gasoline fuels and Diesel fuels in automobiles. Nitrogen oxide emissions contribute, among other things, to increased ozone concentrations at ground level.
In order to counter this critical development, and based on repeated tightening of contaminant limiting values by lawmakers, automobile manufacturers have constantly made efforts to decrease nitrogen oxide concentrations that are created during the operation of a motor vehicle. In this context, a possibility is the application of the SCR method, used in industrial installations, in which ammonia is added to the exhaust gas stream. In this context, the ammonia reacts with the nitrogen oxides to form nitrogen, carbon dioxide and water.
Because of the danger potential relating to the ammonia, carrying ammonia along in a motor vehicle may be problematic. Therefore, it may be possible to produce ammonia, in a quantity exactly required for the chemical reaction, from urea.
This possibility is described, for example, in German Published Patent Application No. 40 38 054, in which an aqueous urea solution is carried along in a container in the motor vehicle, and, with the aid of a hydrolysis catalyst, is split into ammonia and carbon dioxide. In practice, however, various problems come about from the use of an aqueous urea solution. Carrying along aqueous urea not only assumes a corresponding space availability for the tank required for this, but also increases the overall weight of the motor vehicle. Additional disadvantages come about with reference to the wintertime suitability of a vehicle, because of the relatively high freezing point of the urea solution. Besides, in the operation of the motor vehicle, the water proportion of the urea solution has to be evaporated, so that this energy is no longer available to increase the reaction temperature. Also, the production of aqueous urea solutions is expensive, since they are made using deionized water so as to avoid deposits.
The use of dry urea for producing ammonia has been considered, which, after it has been brought to a powdery consistency, is transported to the place of application using a carrier air stream. However, the assumption for this is that the urea is in a free-flowing condition. However, this property is greatly impaired if the dry urea is exposed to moisture, high temperatures or mechanical pressure, since then the baking together of the urea particles may occur. Additional problems come about while transporting solid substances by their inclination to form bridges, which may later cause clogging.
For example, European Published Patent Application No. 0 615 777 describes a method and an appertaining device in which urea is supplied from a storage vessel or reservoir, using a precision dosing unit, to a carrier air stream. The precision dosing unit works according to the principle of a feeding screw, a change in dosing being achieved via a change in the rotary speed of the feeding screw. The solid urea is either already present in powder form in the storage vessel or, if bigger particles are being used, is conveyed to a millwork before being transported. In order to prevent the absorption of moisture, it is suggested there that one should pack the urea under the exclusion of humidity of the air, and to open the package only after inserting it into the storage vessel.
The use of loads of urea that are packed in an air-tight manner may be very expensive, since the urea first has to be packed in an air-tight manner, excluding moisture. In addition, the individual packages must not be too large, since in the course of time the urea absorbs moisture, even in the storage vessel, because of its hygroscopic properties. However, smaller urea portions call for frequent refilling of the storage vessel, which is of little convenience to the user of a motor vehicle.
In order to overcome these aspects, German Published Patent Application No. 197 54 135 describes carrying the urea along in a solid monolithic structure. Depending on requirements, using a removing device, the appropriate quantity of urea is continuously removed from a block, if necessary, the urea is finely ground if the particles are still too big, and the powdery urea is then fed to a carrier gas stream for transport. The removing device is a rotating disk or roll fitted with bristles, abrasive grains, knives or milling tools. By changing the advancing speed of the removing device with respect to the urea block, the dosing quantity may be varied.
Using this procedure, the problem of baking together of the particles is solved, however, other problems remain. Thus, the prepreparation of the urea to form monolithic blocks may be necessary, which means a corresponding preliminary expenditure. An additional disadvantage comes about due to the use of the removing device described there. During removal from the urea block, unavoidably urea particles of different sizes may be produced. This has the result that the urea quantity supplied to the system varies as a function of the particles. Exact dosing in accordance with instantaneous requirements, if at all possible, may be done only within wide boundaries.
German Published Patent Application No. 197 54 135 describes using an additional millwork by which the particles removed from the urea block are milled down to a powder. This, however, may have the disadvantage that the conversion of the urea block to powdered urea represents an additional preinserted method step which may negatively influence the reaction time of the overall system, that is, the system may become too inert. Because of that, the requirement for ammonia conditioned upon the load change may not be able to be satisfied in the short run, or, in the short run, an oversupply may be created.